In recent years, wireless communication techniques have remarkably advanced and with the advancement of the wireless communication techniques, wireless communication terminals such as mobile phones have greatly advanced as well.
Here, in the wireless communication system including the wireless communication terminals, with a high-speed move of the wireless communication terminals, Rayleigh fading is caused and a received signal level fluctuates to a great extent of about 50 to 60 dB. This fluctuation causes problems of demodulation error and degradation in error rate due to the increase in quantization error at the time of AD conversion. Because of this, for wireless communication devices, an automatic gain control device is required to compensate for change in received signal levels.
Broadly speaking, among automatic gain control methods adopted in the automatic gain control devices, feedback gain control method and feedforward gain control method are known.
Since the feedback gain control method controls so as to make deviation between a control target value and a controlled actual value zero, therefore while the method has the advantage of high accuracy, it has the drawback of slow response speed because control operation starts only after the deviation is generated. Japanese Laid-open Patent Publication (JP-P) No. 5-199137 discloses a method for controlling a time constant of a main signal system AGC driver circuit by detecting fluctuation of wireless received signal level in an electric field sudden fluctuation detection circuit. However, this technique in the JP-P No. 5-199137 adopts the feedback gain control method and thus there is an inherent fundamental problem of the feedback gain control method that the response speed is slow.
On the other hand, the feedforward gain control method has a merit of fast response speed, since it can detect fluctuations of received signals at once by using a received signal strength indication (RSSI) and perform gain control. However, the characteristic of the received signal strength indication (RSSI) often does not match the characteristic of the automatic gain control, and thus there is a problem that performing control with high precision is difficult.
Japanese Laid-open Patent Publication No. 8-274558 proposes a feedforward gain control method employing double loop control in which two variable gain amplifiers are arranged in series and a logarithmic amplify detecting output in a single received signal strength indication detecting circuit is inputted into two time constant circuits having two large and small time constants, to control a gain of two variable gain amplifiers. The gain control method in this JP-P No. 8-274558 has a merit of fast response speed since it is basically a feedforward gain control method. However, since an AGC control loop with a large time constant is set in the front and an AGC control loop with a small time constant is set in the back, thereby sudden fluctuations of received signal level pass through the front and causes large fluctuation of input amplitude in the back, and thus there still remains the drawback of the feed forward gain control method that it lacks stability in control.
In addition to the above-described JP-P Nos. 5-199137 and 8-274558, techniques such as the Japanese Laid-open Patent Publication Nos. 2003-78469, 2001-189692, 2004-64525, and 2002-290176 are also disclosed. However, the characteristic of JP-P No. 2003-78469 lies not in the control of AGC time constant, but in a receive-timing detection circuit. Also, the technique in JP-P No. 2001-189692 aims to perform gain control with great precision, by removing an interference signal I from received signals at the time of performing the gain control and by controlling the gain only with hope S wave and noise N wave, which is nothing but the control of gain control value. JP-P No. 2004-64525 also aims to control the gain control value in the same way as in JP-P No. 2001-189692, by employing the feedback method and the feedforward method together. Further, a technique disclosed in JP-P No. 2002-290176 is an idea for obtaining a gain control value to perform AGC by an overflow/underflow detection section and an UP/Down counter. In this way, the techniques in these JP-P Nos. 2003-78469, 2001-189692, 2004-64525, and 2002-290176 should not be compared directly with the technique of the present invention to be described later.